First we need to figure out how much 1 pound of apples would cost. This can be done by dividing 90 cents by 3 pounds.
3 pounds / 90 cents = 1 pound / 30 cents
Now since 1 pound of apples is 30 cents or $0.30 (read 30 cents per pound) we can easily find how much 10 pounds of apple would be.
10 pounds × 30 cents / 1 pound
As you can see the pound units will cancel out leaving only the unit cents.
10 × 30 cents / 1 which is just 10 × 30 cents
And that is 300 cents. Which is of course 3 dollars.
$3.00